Conceptual model of the Tatun geothermal system, Taiwan Patrick Dobsona Erika Gasperikovaa Nicolas Spychera Nathaniel J. Lindseya1 Tai Rong Guob Wen Shan Chenb Chih Hsi Liub Chun-Jao Wangc Shyh-Nan Chenc Andrew P.G. Fowlerd2 Abstract The Tatun geothermal system, located in northern Taiwan, is hosted by the Plio-Pleistocene Tatun volcanic group, consisting dominantly of andesitic lavas, domes and pyroclastic flows. During the late 1960s and early 1970s, geologic mapping, geochemical sampling, geophysical surveys, and the drilling of numerous exploration and temperature gradient wells were conducted at Tatun under the leadership of the Mineral Research and Service Organization (MRSO), the predecessor to the geothermal group at the Industrial Technology Research Institute (ITRI). Deep (1–2 km) wells encountered commercial temperatures (200–300 °C) in the Matsao area, but these initial exploration efforts were discouraged by the presence of very acidic (pH <3.5) and highly corrosive fluids. Numerous geoscience studies conducted over the past 15 years at Tatun have shown that the central portion of the system is dominated by corrosive volcanic fluids, as evidenced by abundant SO2 as well as HCl. More reduced gases are encountered on the flanks of the system, suggesting that water-rock interaction may have at least partly neutralized the volcanic-derived fluids on the margins of this geothermal system. Scientists from Lawrence Berkeley National Laboratory (LBNL) and ITRI have been working together to develop an updated evaluation of the Tatun geothermal system, with the goal of evaluating the geothermal resource potential of the NE margins of the Tatun geothermal system (Zone C), just outside of Yangmingshan National Park. This study provides a review and analysis of existing and new fluid geochemistry data, describes new magnetotelluric data and associated 3-D modeling conducted at LBNL, and incorporates these data and analyses with information from previous studies to create an updated conceptual model for the Tatun geothermal system, with a focus on the Zone C region. These data all suggest that the Zone C area is located on the margin of the main Tatun geothermal system. An exploration well drilled by ITRI will provide important constraints regarding the thermal regime and fluid compositions for this marginal portion of the system. 1. Introduction The Tatun geothermal system is hosted in the Tatun Volcano Group, located in the northern part of Taiwan (Fig. 1). The area was seen as a promising target for geothermal exploration due to the abundance of hot springs and fumaroles associated with young volcanic rocks. The Mineral Research and Service Organization (MRSO) and the Chinese Petroleum Corporation (CPC) conducted extensive exploration efforts at Tatun from the late 1960s until the early 1980s. This work included geologic mapping, geochemical sampling, geophysical surveys, and the drilling of more than 20 exploration wells (>500 m deep) and even more shallow (<500 m) temperature gradient wells (MRSO, 1969, MRSO, 1970a, MRSO, 1970b, MRSO, 1970a, MRSO, 1970b; Feng and Huang, 1970; Chen, 1970; Chen and Wu, 1971; Ellis, 1973; Lan et al., 1980; Chen and Yang, 1984). Five deep (570–1498 m) wells drilled by MRSO succeeded in encountering commercial temperatures (200– 300 °C) in the Matsao area (Fig. 2), but encountered very acidic fluids that corroded the casing and well heads. Fig. 1. Geologic map of the Tatun volcanic center, NE Taiwan. Red dots indicate the location of deep temperature gradient and exploration wells. Red star denotes the location of the ITRI exploration well in Zone C (the results of this well, drilled to assess the geothermal viability of this area, are not reported in this paper). Axis coordinates are in UTM (TWD97 datum, Zone 51R (North)) – northing and easting values are in m. Cross section A-A’ is depicted in Fig. 30, cross section B-B' in Fig. 27, and cross section C-C' in Fig. 2. Fig. 2. NNW-SSE cross section through the Matsao area (see Fig. 1) depicting the stratigraphy and thermal structure of the Tatun geothermal system. Note that temperatures exceeding 290 °C were encountered in well E208 at depths less than 2 km. Adapted from Lee et al. (1994). Following the work conducted by MRSO, the CPC drilled three deep exploration wells. The CPC-MT-1T well was drilled in the Matsao area in 1981–1982 to a depth of 1717 m and encountered a sequence of volcanic lithologiesdown to a depth of 1626 m, where the sedimentary Wuchihshan Formation (Lower Miocene quartz sandstone) was encountered. This well encountered bottom-hole temperatures just above 200 °C. The corresponding alteration mineralogy for this well (with an assemblage of epidote, actinolite, and chlorite below 1100 m) appears to indicate temperatures that were hotter (∼300 °C) than are currently encountered (Chen and Yang, 1984). The CPC-SHP-1T well, located near Szehuangtzeping, was drilled to a depth of 2 km (unpublished CPC report). This well only encountered temperatures slightly above 160 °C, suggesting that it marks the outer margin of the Tatun hydrothermal system. The CPC-CSN-1T well at Chinshan was drilled in 1980 to a depth of 2001 m (unpublished CPC report). This well encountered an upper (∼70 m thick) interval of beach sands and terrace deposits that overlie a ∼140 m thick section of the Muchan Formation (Lower Miocene sandstone/shale), followed by a very thick (>1500 m) section of the quartz-rich sandstone of the Wuchihshan Formation. The temperature in the CPC-CSN-1T well showed a reversal at a depth of around 300 m (with a corresponding estimated local temperature maxima of ∼103 °C), indicating the presence of outflow, and maximum values at total depth less than 150 °C. These early studies confirmed that temperatures greater than 200 °C could be found in the central portion of the Tatun geothermal system at depths less than 2 km. However, severe corrosion issues that occurred with these wells raised the question of whether or not near-neutral fluids could be encountered in the deep reservoir. The recommendations of White and Truesdell (MRSO, 1970b) included the following suggestion: “The most attractive possibility for geothermal resourcesmay be large reservoirs (2 sq. km or larger) in Miocene sandstones below the volcanic cover. The most important questions concern the depth of cover, the size and permeability of the reservoirs, and, especially, the primary acidity of the deep fluids. Our available evidence suggests that the primary acidity may be intolerably high (pH about 2?) especially near the fumarolic areas. If primary acidity is very high, we regretfully conclude that any deep resources are economically unattractive under present conditions. If, in contrast, the deep fluid is not too corrosive, we strongly urge a vigorous effort to discover large reservoirs by geophysical methods and deep drilling. If the deep water at Matsao is too acid by only a small margin, drilling further away from a fumarolic area may produce deep water with more of its primary acidity neutralized (1/2 to 1 pH unit higher), which may be acceptable for development.” The objective of the present study is to reevaluate the viability of developing the Tatun geothermal system. Since the early drilling work was conducted at Tatun, much of the area has been transformed into the Yangmingshan National Park. Any new drilling must take place outside of the current park boundaries. While the central portion of the Tatun system appears to have fluids that are too acidic for commercial development, variations in gas geochemistryobserved in the fumaroles in the NE margins of the Tatun geothermal system (hereafter referred to as Zone C) appear to indicate a decrease in volcanic signature, with fluids that are more geothermal in character. This work provides an updated conceptual model of the Tatun system that examines the NE sector of Tatun as a possible exploration target. 2. Geology of the Tatun area In order to develop a conceptual model for the Tatun geothermal system, it is critical to understand the geologic and tectonic framework of the area. The Tatun region has undergone a complex tectonic and structural history. Pliocene-Quaternary volcanism at Tatun is interpreted to be related to lithospheric extension in the N. Taiwan mountain belt, with the mantle source modified by subduction-related processes, and is not thought to be part of the Ryukyu volcanic arc (Wang et al., 1999). While most of Taiwan is currently undergoing crustal shortening, northern Taiwan is experiencing extensional deformation. When the area was under compression, two major thrust faults were formed: the Kanchiao fault (east of Shihlin and Wanli), and the Chinshan fault (west of Peitou and Chinshan) (Chen, 1970). Coincident with the Chinshan thrust fault is the currently active Shanchiao normal fault, which strikes NNE with a dip of ∼60° to the SE (Cheng et al., 2010). This feature appears to bound the geothermal activity at Tatun (Fig. 1), as all of the thermal features are located SE of this fault. Another structural feature, the Chilian lineament, appears to serve as the other bound to the geothermal area. Initial studies of the volcanic history and stratigraphy of the Plio-Pleistocene Tatun Volcano Group (TVG) subdivided the TVG into three major subgroups (Chen, 1970) and further divided the area with geothermal activity into eight volcanic subgroups (Chen and Wu, 1971), noting that the Tatun system consisted dominantly of andesitic lavas, domes and pyroclastic flows. Song et al. (2000) conducted a detailed reevaluation of the Tatun volcanic center, and noted that it consists of around 20 volcanic centers (cones and domes) over an area of about 400 km2. These have been subdivided into six subgroups: the Chutzeshan, Tatunshan, Chihshinshan, Neiliaoshan, Huangtsuishan, and the Tinghuohsiushan volcanic subgroups. These centers are in two spatial clusters: one along the Chinshan fault, and the other on an ENE to NE trend east of the Chinshan fault.